In a communications system operating according to the Orthogonal Frequency-Division Multiplexing (OFDM)-based Long Term Evolution (LTE) standard, data is transmitted between user equipments and radio base stations (referred to as evolved Node Bs, e-NodeBs) on a number of frequency resources which can be dynamically allocated to different communication sessions. A user equipment (UE) can request scheduling of resources for transmission of uplink (UL) data by sending a Scheduling Request (SR) to the e-NodeB to which it is currently connected. The e-nodeB will then respond to the scheduling request by sending a UL grant message to the user equipment, the UL grant message containing information on a frequency or frequencies on which the uplink data is to be transmitted (the timing of the scheduled resources for transmission of uplink data is typically implicitly given by the timing of the receipt of the UL grant message). The uplink data can then be sent by the user equipment on the scheduled time/frequency resources.
When a user equipment becomes active in an LTE system, the user equipment will typically be allocated a dedicated Physical Uplink Control Channel (PUCCH), which is a dedicated control channel on which inter alia scheduling requests may be transmitted to the e-nodeB. According to the current LTE standard, the power at which a scheduling request will be transmitted on the PUCCH channel is determined according to an expression provided in the standard (see 3GPP Technical Specification (TS) 36.213, version 8.2.0, section 5.1.2):PPUCCH(i)=min{PMAX,PO_PUCCH+PL+ΔTF_PUCCH(TF)+g(i)}[dBm]  (1)where                P0_PUCCH consists of a cell-specific parameter P0_NOMINAL_PUCCH, which is broadcasted (in System Information Block 2, SIB2), and a UE-specific parameter P0_UE_PUCCH, which is sent to the user equipment through dedicated signalling over the Radio Resource Control (RRC) protocol. P0_PUCCH is a semi-statically configured parameter that is updated from time to time, typically on a time scale of hours. P0_PUCCH is used to compensate for interference, but is not able to follow interference changes that are faster than the P0_PUCCH update time scale.        PL is the UE's estimate of its own pathloss.        ΔTF_PUCCH(TF) corresponds to a transport format specific factor signalled through RRC. ΔTF_PUCCH(TF) takes the same value for all scheduling request transmissions in a cell, but could take a different value when other types of information is transmitted over the PUCCH; and        g(i) represents absolute or accumulated Transmit Power Control (TPC) commands received by the user equipment from the e-nodeB (in Dedicated Control Information (DCI) formats 1, 2 or 3). TPC commands are used to adjust the transmission power of a user equipment in order to compensate for variations in noise and interference levels in a cell.        
Hence, the transmission power at which scheduling requests will be transmitted on the PUCCH channel depends on the semi-static parameter P0_PUCCH, the pathloss estimation PL made by the user equipment, the transport format specific compensation ΔTF_PUCCH(TF) and the TPC command(s) received from the e-nodeB. Expression (1) has been derived in order to ensure that a user equipment transmits at an appropriate power so that interference is kept low while sufficient quality of service is maintained.
However, there might be situations where the transmission power given by expression (1) is not sufficient, so that an e-nodeB to which the user equipment is currently connected fails to hear a scheduling request transmitted on the PUCCH channel. This can for example be the case when the interference in the cell in which the user equipment is active changes on a timescale that is shorter than the timescale of the transmission of TPC commands from the e-nodeB, or in the absence of TPC commands on a timescale shorter than the timescale of P0_PUCCH updates. TPC commands are typically transmitted to the user equipment when there is dynamically scheduled downlink data to be transmitted to the user equipment. If there has been no dynamically scheduled downlink data to be transmitted to the user equipment for some period of time, it may be that the latest transmitted TPC command(s) do(es) not provide adequate compensation for the current interference in the cell.
If the combination of P0_PUCCH and any TPC command(s) is not an adequate compensation for interference plus noise, the user equipment will re-send its scheduling request, again and again using the same power, until the user equipment gets a grant. In unfortunate circumstances, the user equipment may end up in a more or less indefinite loop of sending scheduling requests that will never be heard by the e-NodeB. A similar situation may occur if the scheduling request fails for some other reason, for example if the user equipment fails to transmit correct scheduling requests, or transmits them on the wrong resource.
As can be seen from the above, there is a need to improve the robustness of the scheduling request procedure in mobile radio communications systems operating according to the LTE standard.
This need has been addressed in the standardisation proposal R2-083436, 3GPP TSG-RAN WG2 #62bis, wherein it is disclosed that the problem of endless scheduling request transmissions may be solved by re-using a recovery procedure which was initially standardised to stop endless attempts to transmit on the Random Access CHannel (RACH). In this solution, the Medium Access Control (MAC) protocol indicates to the RRC protocol when a problem with scheduling requests on PUCCH has been identified, based on a timer or a counter. When such a problem has been identified, RRC starts a timer referred to as T312. If a scheduling request is successfully received by the e-NodeB before the timer T312 has expired, MAC informs RRC. However, if no recovery has occurred at the expiry of the timer T312, RRC will take further action following the specification for radio link failure, used for example to handle random access channel failure. This is further described in TS 36.331 v8.2.0, Section 5.3.10 “Radio link failure related actions”.
Although the method described in standardisation proposal R2-083436 will ensure that a user equipment will never end up in a situation where it is transmitting an endless number of scheduling requests that will never be heard, it does so at great expense. To perform the procedure standardised for the expiry of T310 is time consuming and requires a large amount of signalling.